The rubber and plastics industries for years have used reinforcing fabric members in conjunction with flexible polymers to produce reinforced flexible composite materials to fabricate a variety of mechanical goods such as tires, hose, liners, diaphragms, seals, gaskets, protective fabrics, tapes, accumulators, airbags, fuel cells, fuel tanks, flexible couplings, medical tubing, structural bearings and so forth.
The purpose of reinforcing members in reinforced flexible materials is to limit deformation of the composite materials, to satisfy requirements of the intended service, to strengthen the composite, to maintain dimensional stability, to absorb energy and to extend service life.
Over the years, improved reinforcing fabrics have progressed through cotton, rayon, nylon, polyester, glass, steel, polyaramides and polytetrafluoroethylene (PTFE) fibers. These materials have been used in the form of individual fibers or cords and in the form of a woven, knitted or braided fabrics. Each of these reinforcing fabrics contributes unique attributes to the reinforced flexible composite material's physical properties and each suffers from limitations in certain critical physical properties such as strength, elongation, stiffness, flexibility, fatigue resistance, creep resistance, and chemical and thermal stability.
A particular advance in the use of fabrics to reinforce composites was taught in U.S. Pat. No. 2,772,444 to Burrows, et al. outlining the use of PTFE in oriented fiber form. Industry has investigated the use of PTFE in oriented fiber form in composite structures, however, because of its low tensile strength, poor creep resistance and high cold flow, its use as reinforcement in flexible composites has been very limited. U.S. Pat. No. 3,513,064, to westley, et al. discloses the use of PTFE fiber in conjunction with a fluoroelastomer and a fluoroplastic for use in protective clothing where the low physical properties of the fabric are adequate for the end-use.
One area where reinforced composites have demonstrated utility is in diaphragms for use in valves and for pumps. In the past, a wide range of flexible polymers combined with reinforcing fabrics have been used. The identity of the particular materials used is dependent on the end use of the diaphragm, however, when a diaphragm is to be used in areas requiring chemical and thermal resistance, fluoropolymers, such as fluorinated thermoplastics consisting of copolymers of tetrafluoroethylene, copolymers of vinylidine fluoride, copolymers of chlorotrifluoroethylene, are typically used. These materials exhibit a good degree of chemical resistance and are reasonably flexible and tough over a wide temperature range. However when more hazardous chemicals are present and greater environmental resistance is needed, polytetrafluoroethylene (PTFE) is the material of choice due to its high molecular weight and high degree of fluorination both of which impart chemical resistance to the resultant diaphragm. However, PTFE does not possess the physical properties of the other flexible polymers used in diaphragms, and if used as a single diaphragm, a greatly reduced flex life may result.
Normally to use a PTFE diaphragm, diaphragm pumps for pumping hazardous chemicals are fitted with two diaphragms; one diaphragm comprised of a fluorocarbon resin to resist degradation by the hazardous chemicals, and a second diaphragm comprised of fabric-reinforced elastomer to withstand the mechanical, pneumatic or hydraulic forces needed to drive the pump. However, a two diaphragm system does have weaknesses. Pumps fitted with two diaphragms require additional hardware, as compared to pumps fitted with one diaphragm, to maintain the diaphragms in the proper orientation with each other. The diaphragms must be fitted in the proper orientation and in the proper order for the pump to operate. Another problem with a two diaphragm system is that the two diaphragms may become separated from one another and form a gap or cavity between the diaphragms so that continued operation of the pumping means creates a negative pressure between the two diaphragms. When negative pressure is created between the two diaphragms, there is a tendency for material to be sucked between the diaphragms. This may result in premature failure of the diaphragm system and/or contamination of the material being pumped. If the material sucked between the two diaphragms is hazardous in nature, the hazardous material may issue from the pump creating an unacceptable risk to personnel in the vicinity of the pump. Also, two diaphragm systems are difficult to install. Typically, two diaphragm systems require the two diaphragms to be torqued together at installation as well as at some period of time after installation since deformation experienced by the PTFE diaphragm under the load applied would not be identical to the deformation experienced by the fabric-reinforced elastomer diaphragm resulting in a loss of compressive force between the two diaphragms. Also, forces generated by a driving means and transmitted by the diaphragm comprised of fabric-reinforced elastomer may not be evenly distributed over the PTFE diaphragm thereby causing a premature failure of the diaphragm system.
It is to the production of a composite diaphragm, having a PTFE face securely attached to a flexible backing that the present invention is directed.